Descriptions

The comparative values of impedance and resistance to reactance
ratios are important parameters in determining the load
capacity of an interconnection of power transformer, called a bank.
The full design capabilities of parallel or three-phase connected
transformers cannot be utilized unless the following constraints are
applied.
1. All transformer voltage ratios must be identical.
2. All transformer percent impedances must be equal.
3. The resistance to reactance ratios of all transformers
must be equal.
Departure from these conditions involves either an uneconomical
division of current, or a circulating current, both of which will
lower the efficiency and decrease the load that the bank can carry
without overheating.
A transformer bank whose individual transformers do not have
equal percent impedances will have a load distribution that is unbalanced.
The transformer with the lowest percent impedance will
supply its full-rated MVA capacity while the other transformers are
underloaded. This condition represents a loss in the capacity of the
transformer bank.
Transformers having widely different impedance values can be
made to divide their load in proportion to their rating by placing the
proper impedance in series with those transformers that have low
impedances.
The replacement of a damaged transformer in a bank will produce the best transformer load distribution if the following constraints
are applied:
1. The transformer should have impedance and voltage ratio
equal to those of the existing transformers in the bank.
2. The voltage ratios and the percent complex impedances
should be equal to those of the existing transformers
even though the KVA ratings are not equal.
3. When the resistance and/or the reactance values are
different, they should be larger than those values of
the existing transformers.
A difference in the voltage ratios of paralleled or banked transformers
will produce a much greater reduction in load capacity than
mismatched impedances. This condition should be avoided if at all
possible.